The invention relates to the control of electric motors, and in particular the control of electric motors of wound-rotor synchronous type.
An electric motor of wound-rotor synchronous type comprises a fixed part known as the stator and a moving part known as the rotor. The stator comprises three coils offset by 120 and powered with alternating current. The rotor comprises one coil powered with direct current. The phase currents of the stator depend on the resistances and inductances of the rotor and the stator as well as the mutual inductance between the rotor and the stator.
Conventionally, control signals are used to control the currents of the stator and the rotor. In this way regulation is implemented which depends on the measured values of the currents.
If one of the sensors supplying these measured values is faulty, for example constantly indicating a zero current value, the regulation will increase the value of the control signals in order to increase that of the current. Due to this, the value of the control signals can diverge until maximum values are reached, which causes the appearance of particularly high, or even destructive currents. By way of example, it is possible to reach control signal values of up to 400 Volts which cause the appearance of currents of up to 40 000 Amps. Of course, such currents are destructive for the transistors used for control, for example Insulated Gate Bipolar Transistors (IGBT).
Note that the Japanese patent application JP 2001 268980 and the U.S. Pat. No. 5,047,699 have proposed the regulation of direct currents by means of an electrical machine model.
The reader is also referred to the document US 2002 0008492 which describes a method for detecting sensor faults.